1. Field of the Invention
This invention is in the field of permanent magnet holding plates. A primary use is as a magnetic holding saddle for a printing press roll or cylinder for holding a flexible, steel-backed printing plate in place during the printing operation.
2. Description of the Prior Art
A prior art magnetic cylinder or saddle for a printing press is disclosed in U.S. Pat. No. 3,810,055 by Theodore D. Wright. Although the U.S. Pat. No. 3,810,055 device can be used for other purposes, it is primarily for use as a magnetic holding device to hold a flexible or curved steel-backed printing plate without the need for mechanical clamp or the like which has been necessary in the past. The device described in the '055 patent is constructed by spiraling a pair of elongated strips of flexible permanent magnets in a pair of parallel spiraling grooves around a cylinder. Between the magnet strips are strips of magnetic material and the permanent magnets induce magnetic poles into the magnetic material strips so they become pole pieces and concentrate the magnetic lines of force near the surface of the cylinder to create a strong magnetic holding force to hold the printing plate in place on the cylinder. Another device which finds its principal use as a magnetic holder for a printing plate is shown in U.S. Pat. No. 3,824,927 by Pugh et al. Here the magnetic holding device is formed by a sheet or layer of magnetic rubber placed over a curved steel supporting layer.
While both of the aforementioned devices may work well and may be meritorious, they do have certain limitations. One difficulty with the U.S. Pat. No. 3,810,055 device is the problem of making the spirally-wrapped cylinder into a saddle by cutting the cylinder into sections. Another difficulty is the problem that sometimes occurs in assemblying the device in handling the long strips of flexible permanent magnets to arrange them in the slots. Also, there can be some difficulty in ensuring intimate contact between the flexible magnet strips and the adjacent rigid pole pieces to reduce magnetic field loss. Another disadvantage of the U.S. Pat. No. 3,810,055 magnetic holder is that because of the way it is constructed, it cannot be made thin enough to be used in certain applications.
The U.S. Pat. No. 3,824,927 device, without pole pieces, would appear to have insufficient mechanical strength and magnetic holding power so would need auxiliary means for holding the printing plate in place, and is not too durable because the layer of flexible magnet material is located unprotected on the outer surface of the cylinder or saddle. This makes it susceptible to damage during use and handling.